Heretofore, piezoelectric elements formed of monolithic piezoelectric substrates in which internal electrodes and piezoelectric layers are alternately stacked have been known.
Patent Document 1 discloses a piezoelectric actuator including such a monolithic piezoelectric element. FIG. 15 is a schematic perspective view of a piezoelectric actuator described in Patent Document 1. The piezoelectric actuator 101 includes a piezoelectric substrate 102. In the piezoelectric substrate 102, a plurality of internal electrodes 103a to 103e and piezoelectric layers are alternately stacked. The internal electrodes 103a to 103e are exposed at all the side faces of the piezoelectric substrate 102.
Insulating layers 104a, 104b, and 104c are formed on a first side face 102a of the piezoelectric substrate 102 to cover the exposed portions of the internal electrodes 103a, 103c, and 103e. Insulating layers 105a and 105b are formed on a second side face 102b opposite to the first side face 102a. The insulating layers 105a and 105b are formed to cover the exposed portions of the internal electrodes 103b and 103d on the side face 102b. 
External electrodes 106 and 107 are vertically formed on the first side face 102a and the second side face 102b, respectively. The external electrode 106 is electrically connected to the internal electrodes 103b and 103d on the side face 102a. The second external electrode 107 is connected to the internal electrodes 103a, 103c, and 103e on the side face 102b. 
Patent Document 2 discloses a piezoelectric actuator having a structure illustrated in FIG. 16. The piezoelectric actuator 111 includes a monolithic piezoelectric substrate 112. The piezoelectric substrate 112 includes a plurality of internal electrodes 113 a to 113e stacked with piezoelectric layers interposed therebetween. Concave portions 112c to 112e are formed at exposed portions of internal electrodes 113a, 113c and 113e on a first side face 112a of the piezoelectric substrate 112. The concave portions 112c to 112e are filled with insulating materials 114a to 114c, respectively. The internal electrodes 113a, 113c, and 113e are exposed at a second side face 112b opposite to the first side face 112a. Concave portions 112f and 112g are formed at exposed portions of the internal electrodes 113b and 113d on the second side face 112b. The concave portions 112f and 112g are filled with insulating materials 115a and 115b. The internal electrodes 113b and 113d are exposed at the first side face 112a. A first external electrode 116 and a second external electrode 117 are formed on the first side face 112a and the second side face 112b, respectively.
In the piezoelectric actuators 101 and 111, an electric field is applied between the first external electrode 106 and the second external electrode 107 or between the first external electrode 116 and the second external electrode 117 to expand or contract the piezoelectric layers in the thickness direction, thus operating the actuators.
A prior art piezoelectric actuator illustrated in FIG. 17 is also known as a monolithic piezoelectric actuator. A piezoelectric actuator 121 includes a monolithic piezoelectric substrate 122. The piezoelectric substrate 122 includes a plurality of internal electrodes 123a to 123e alternately stacked in the thickness direction. Internal electrodes 123a, 123c and 123e protruding from a second side face 122b do not reach a first side face 122a. Internal electrodes 123b and 123d protruding from the first side face 122a do not reach the second side face 122b. Thus, a voltage can be applied to external electrodes 126 and 127 formed on the side faces 122a and 122b to operate the actuator.
In the piezoelectric actuators 101, 111 and 121, internal electrodes 103a to 103e, 113a to 113e, and 123a to 123e are exposed at side faces of the piezoelectric substrate 102, 112 and 122 on which external electrodes are not formed. Thus, when adjacent internal electrodes in the thickness direction are at different electric potentials, migration between the adjacent internal electrodes may occur on the exposed side faces of the piezoelectric substrates 102, 112 and 122.
In these situations, Patent Document 1 proposes a structure in which all the side faces of the piezoelectric substrate are coated with an exterior resin to cover the monolithic piezoelectric substrate 102, in other words, to cover not only portions on the side faces of the piezoelectric substrate 102 at which the internal electrodes 103a to 103e are exposed, but also the first external electrode 106 and the second external electrode 107. In the monolithic piezoelectric substrate 102, each of the first external electrode and the second external electrode have a lead wire. Except for the portions from which the lead wires are drawn, all the side faces of the piezoelectric substrate are covered with an exterior resin.
The exterior resin contains 800 ppm or less of alkali metal and/or alkali earth metal. Since the content of alkali metal and/or alkali earth metal is 800 ppm or less, Patent Document 1 states that the concentration of the alkali metal ion and/or the alkali earth metal ion at the edges of the internal electrodes on the negative electrode side is prevented and the migration is prevented. The resin material containing the alkali metal and/or the alkali earth metal is said to be preferably a fluorocarbon resin or a silicone resin.
Patent Document 2 proposes a structure in which a covering layer is formed to cover the side faces of the piezoelectric substrate 112 of the piezoelectric actuator 111 to prevent migration. Also in Patent Document 2, lead wires are connected to the first external electrode 116 and the second external electrode 117. The covering layer is formed on the side faces other than the portions from which the lead wires are drawn. The covering layer may be formed of at least one selected from the group consisting of copper oxides, nickel oxides, tin oxides, zinc oxides, chromium oxides and iron oxides. It is said that the covering layer provides electrical insulation and can thereby prevent migration.
Patent Document 3 discloses a circuit in which an electric charge leakage means is connected, as an additional component, to electrodes of a piezoelectric actuator placed at different electric potentials. Patent Document 3 states that the electric charge leakage means can prevent a decrease in the degree of polarization of the piezoelectric substrate because of the pyroelectric effect.
Patent Document 1: Japanese Unexamined Patent Application Publication No. 3-12974
Patent Document 2: Japanese Unexamined Patent Application Publication No. 2003-17768
Patent Document 3: Japanese Unexamined Patent Application Publication No. 60-249877